As shown in FIG. 1, a general electron gun for a cathode ray tube is structured in such a way that a cathode 2, a control electrode 3, and a screen electrode 4 which together constitute a triode for generating beams, and a focus electrode 5 and a final accelerating electrode 6 which focus and accelerate the electron beam by forming a main lens system wherein the focus electrode 5 and the final accelerating electrode 6 are sequentially arranged in the direction of the electron beam's path. In the electron gun 1 having the above constitution, a thermal electron emitted from the cathode 2 is formed as a beam by being previously focused and accelerated through a prefocus lens 5A positioned between the screen electrode 4 and the focus electrode 5, and this electron beam arrives on the phosphor screen by being finally focused and accelerated through the main lens 5B positioned between the focus electrode 5 and the final accelerating electrode 6. Such an electron beam is continuously projected onto the phosphor surface, in which the beam sequentially scans the desired positions by the deflection of the magnetic field, reproducing a completed image on the phosphor surface. To obtain a sharp image having a high resolution on the phosphor surface, the diameter of the beam spot formed on the phosphor surface is as small as possible, and around the beam, the spot's halo due to the influence of the spherical aberration should be minimal.
The aforementioned conventional electron gun has a very strong main lens because of its structural characteristics. Accordingly, the intensity of the beam spot formed in the phosphor surface and that of the spot halo around the core of the beam spot are relatively high due to the strong influence of the spherical aberration to the electron beam passing through the main lens, so that a high quality picture is unattainable.
To solve the problem of deteriorating the beam spot's characteristics due to spherical aberration, a larger-aperture main lens should be provided in the electron gun. To accomplish this in the conventional electron gun, the electron beam passing holes of the focus electrode and those of the final accelerating electrode have maximum-sized diameters. But, since the size of the electron gun is limited by the diameter of the funnel's neck (where an electron gun is disposed), the diameter of the beam passing holes are limited.
That is, the electron beam passing holes are formed in an in-line manner in the focus electrode and the final accelerating electrode, which are inserted into the neck of a cathode ray tube, so that each diameter of the electron beam passing holes is smaller than the distance between the centers of two adjacent electron beam passing holes. Also, if the distance between the centers is enlarged so that it is larger than the original designed value, the convergence degree of the outer electron beam of the electron gun becomes larger, thereby deteriorating the picture quality.
The U.S. Pat. No. 4,370,592 discloses a method to solve these problems. As shown in FIG. 2, the u-shaped portions (hereinafter referred to as rims 7' and 8') are recessed by a predetermined depth in the outgoing side 7A of the focus electrode 7 and the incoming side 8A of the final accelerating electrode 8, thereby forming large-aperture electron beam passing holes 7H and 8H through which R, G, and B electron beams pass, and R, G, and B independent small-aperture electron beam passing holes 7H' and 8' on the bottom of the large-aperture electron beam passing holes 7H and 8H.
In such an electron gun, since the large-aperture electron beam passing holes 7H and 8H are asymmetric, the electron beam having passed through the central indepentent small-aperture electron beam passing hole and the electron beams having passed through the outer independent small-aperture electron beam passing holes are differently affected by the vertical and horizontal focusing forces which influence the formation of the electron beam spot formed on the phosphor surface.
That is, as shown in FIG. 2B, the outer electron beams RB and BB passing through the large-aperture electron beam passing hole of the focus electrode 7 or the final accelerating electrode 8, pass near the rims 7' and 8' maintaining a low voltage or a high voltage in the horizontal direction; and the central electron beam GB passing through the central electron beam passing hole passes a relatively long distance from the rims 7' and 8'. Accordingly, the outer electron beams RB and BB are relatively strongly focused in the horizontal direction and the central electron beam GB is relatively weakly focused. Also, the distance between outer electron beams RB and BB and the rims 7' and 8' in the vertical direction are almost equal to that in the horizontal direction. Accordingly outer electron beams are affected by the strength of the focusing force in the vertical direction which is similar to that in the horizontal direction.
However, since the distance between the central electron beam GB and the rims 7' and 8' in the vertical and horizontal directions are different and the distance to the rim in the horizontal direction is relatively large, the central electron beam is affected by a strong electric field in the vertical direction. Consequently, the central electron beam is affected by a relatively stronger focusing force vertically than the horizontally.
Accordingly, the outer electron beams RB and BB and the central electron beam GB having passed through the main lens have cross-sections of different formations, respectively, so that an evenly shaped beam spot formed on the phosphor surface cannot be obtained.